By using Koster-Slater Green's function method and on-site defect potential approximation, a systematic method of A1 symmetric deep level wavefunction calculation of substitutional defect pairs is worked out. The method is applied to the spectator-oxygen pairs in GaP successfully. The Zn-O and Cd-O pair wavefunctions are obtained for the first time. The wavefunction is mainly concentrated on four bonds which locate at four nearest neighbour sites of oxygen center and point toward the oxygen. Nevertheless, the four bonds are not in the equal weight. The component of the bond which links spectator with oxygen is reduced in comparison with other three and it is more reduced for more electropositive spectator. The rest part of the wavefunction extends diversely over a wide range of space.